1. Field of the Invention
The present invention relates to a flow control device of a power steering apparatus suitable for use in vehicles and the like. More particularly, the present invention relates to a flow control device of a power steering apparatus capable of saving energy by decreasing flow rate of operating fluid supplied from a pump to a control valve at low load pressure.
2. Description of the Prior Art
Flow control devices of power steering apparatuses have been proposed for saving energy of vehicle engine and for providing rigidity of steering operation. The energy of vehicle engine is saved by decreasing flow rate of operating fluid supplied from a pump to a control valve when load pressure caused by rotation of a steering wheel is low, i.e., the steering wheel is not rotated. The rigidity of steering operation is provided by restricting the flow rate when the pump is driven at very high speed, i.e., the vehicle runs at very high speed.
An example of the flow control device is shown in the Japanese Unexamined Patent Publication No. 8-282513. The flow control device has an energy saving mechanism acting in accordance with the load pressure. The energy saving mechanism includes a movable member located behind a bypass spool through a spring urging the bypass spool. The bypass spool has a control rod for regulating an opening area of a metering orifice.
Since the movable member is slid toward the bypass spool by a rise of the load pressure, initial load of the spring is increased, so that the bypass spool is slid to increase the flow rate supplied to a control valve. The rise of the flow rate causes assistance force for a power steering apparatus when the steering wheel is rotated. Therefore, the energy saving mechanism decreases the flow rate supplied to the control valve for saving energy of a vehicle engine, since the movable member is slid in response to the load pressure.
However, the flow control device with the energy saving mechanism involves some drawbacks described hereinafter. The flow control device needs a special housing different from a general housing of a general flow control device with no energy saving mechanism because the energy saving mechanism is located behind the bypass spool in the flow control device for saving the energy. Die to the special housing, the cost of the flow control device with the energy saving mechanism is relatively high.
In addition, it is difficult to decide the spring constant of the spring arranged between the bypass spool and the movable member, since the stiffness of the spring is changed by the movable member slid by the load pressure.